Defect Equilibration and Metastability in Low-Spin-Density a-Si:H
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DEFECT EQUILIBRATION AND METASTABILITY IN LOW-SPIN-DENSITY a-Si:H T.J. MC MAHON Solar Energy Research Institute, 1617 Cole Boulevard, Golden, CO 80401
ABSTRACT To first order, the relaxation kinetics of thermally generated defects with spin observed in two differently prepared, 60-jim-thick undoped hydrogenated amorphous silicon (a-Si:H) films are consistent with a two-level system having a formation energy of 0.35 eV and an anneal barrier of 2.1 eV. However, closer examination of how relaxation depends on thermal treatments reveals the complexity that might be expected from a disordered material. For example, the stabilization of many spins quenched in from 260'C can be increased by annealing at an intermediate temperature: It appears that, some 260'C defects equilibrating further at 205*C will relax and become more locked-in configurationally than defects simply equilibrated at 260'C. Crossover of annealing data is the result. Crossover cannot be explained with twolevel system approaches. Models in which a spin can be stabilized with alternate structural configurations must be invoked. INTRODUCTION AND BACKGROUND Generation and annealing of thermal defects are studied by electron spin resonance on two differently prepared, low-spin, 60-jim-thick undoped hydrogenated amorphous silicon (aSi:H) samples. Glasstech Solar, Inc. has prepared a high frequency(HF) film by rf glow discharge at 110 MHz with H-dilution and a low frequency(LF) film at 13.6 MHz from pure silane. Previously, the time constants for annealings of 250'C fast-cool defects in the HF film were measured and found to be activated with an anneal barrier of 2.1 eV [1]. Equilibrium spin densities were also activated with a formation energy of 0.35 eV [1]. When annealings of defects quenched in from high and low temperature are compared, the defects introduced at the higher temperature always annealfaster; the metastable states with higher formation energies have smaller annealing activation energies. To first order, the spin densities and relaxation kinetics of the two films are very similar. Defect relaxation observed by electron spin resonance fit a stretched exponential (SE) time dependence. At least three distinct models are available to explain SE relaxation behavior in aSi:H. (i) The hydrogen-diffusion controlled relaxation (HCR) model was first proposed. It uses the dispersive diffusion of hydrogen to limit the annealing of shallow occupied band-tail states in doped films [2]. Later, annealing kinetics of excess thermal defects with spin in undoped films were described by the trapping of hydrogen into an exponential distribution of weak bonds [3]. In this model, the size of the annealing energy barrier is anticorrelated with the defect formation energy. (ii) One defect controlled relaxation (DCR) model uses an ensemble of two-level, localized defects with an exponential distribution of barriers to explain both the SE relaxation kinetics and the Meyer-Neldel rule [4]. (iii) Finally, the hierarchically constrained dynamics (HCD) model requires that
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